what do i need to make 3rd gen posi and does a 4th gen come stock with posi

 

Not sure how to answer with your 3rd gen posi question. But yes 4th gen rears came stock with posi. Well basically in a sense. Just a limited slip. Which means when you turn it allows the inside wheel to turn faster than the outside wheel keeping it much easier to get the car to go around corners. But if your going in a straight line and floor it both wheels lock up and turn at the same time and same speed. Basically the best of both worlds.